This invention relates generally to paper pulp processing machinery and more particularly to rejects drainers such as those for separating knots or other coarse particles from an acceptable pulp slurry.
Processing of wood pulp for papermaking requires removal of knots and other coarse undigested particles from the pulp slurry. This is commonly accomplished in a knot drainer, in which the pulp slurry is passed through a screen upon which the knots are retained. The knots are scraped or otherwise removed from the screen and discharged from the drainer.
Knot drainers consist usually of either high speed horizontal vibratory generally flat surfaced screens or screw type drainers. The screw type drainers may be either stationary cylindrical screen type or rotary screen type machines, which may have either horizontal or vertical axes of rotation, although the vertical axis is more commonly used today. One such rotary screen type drainer is described in a pending U.S. patent application having Ser. No. 610,696, filed Nov. 8, 1990, now U.S. Pat. No. 5,143,220 and commonly assigned herewith. In the latter case the screen is attached to the outer edge of the knot transport screw flights and rotates with them. The knots travel up the screw flights in response to inertial forces which overcome gravity, hydrodynamic forces, and the friction between the knots and the screw flight. Acceptable fibers pass through the screen perforations so that the knots are ultimately discharged from the drainer in a relatively clean condition. The rotary screen screw type knot drainer provides the advantage of eliminating relative motion between the screw flight and the screen. By comparison with non-rotating screen drainers, the rotating screen drainer has less wear and tear, because it eliminates collisions which would be caused by screen and screw irregularities if the screen were not rotating, and it also eliminates the knot crushing or grinding that would result from relative motion between the screw flights and the screen. Since knots are not crushed or ground, the acceptable fiber slurry, passing through the screen perforations, is not contaminated with knot dirt and ground-off wood particles detrimental to pulp and paper making quality.
Vibratory screen type knot drainers are known to require significant maintenance and repair due to fatigue and wear damage to the vibrating parts and structure. Stationary screen screw type knot drainers experience wear due to contact between irregularities of the screw and the screen as well as the crushing and grinding action already described. In addition, they also yield an increased debris content in the pulp slurry which can ultimately degrade paper quality.
Rotary screen screw type knot drainers experience lower incidence of fatigue damage, lower wear damage as a result of virtual elimination of crushing and grinding action, and, thus, last longer and produce cleaner pulp. One such vertical axis knot drainer has a tangential feed slurry inlet chamber at the bottom, a rotatable screw flight extending generally from the inlet chamber upward to the knot discharge chamber, a rotatable screen basket attached to the lower portion of the rotatable screw flight to define a screening chamber, and a knot washing and liquid separating stationary housing extension communicating between the screening chamber and the knot discharge chamber and encasing the upper extension of the rotatable screw flight.
Simply stated, in this knot drainer, the knot containing pulp slurry is introduced through the tangential inlet into the inlet chamber and flows spirally upward into the screening chamber. The screw conveyor flight transports the knots contained in the pulp slurry through the screening chamber in which the acceptable pulp fibers pass through the perforations in the rotating screen. Above the screening chamber, the screw conveyor flight continues to transport the knots through the fiber wash-off zone and liquid separation and drain-off zone of the stationary housing extension to the knot discharge chamber. The tangential feed is desirable because it promotes centrifugal separation of stones and other heavy "junk" materials that may be included in the feed pulp slurry so that they may be accumulated for ultimate discharge from the knot drainer through a special outlet.
The vertical axis rotary cylindrical screen type knot drainer just described is, however, subject to knot transport interruptions which necessitate shutdowns to clean out the system. It has been determined that, independent of the operating speed of the knot drainer, the pulp consistency, and geometric relationships within the knot drainer, unacceptable knot transport interruptions, with subsequent knot accumulation, occur both in the screening chamber and in the housing extension. These interruptions result in knot accumulation on the screw flights which creates serious dynamic imbalance, can seriously impact the production capacity through the knot drainer unit, and may, thus, require costly maintenance, production downtime and expensive duplication of equipment to maintain production flow during shutdown necessitated by knot transport interruptions.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.